Wireless communications pose many challenges. Among them, multi-path fading, generated as the same signal is received through different paths and implicitly different phases, is perhaps the most problematic. To address this issue, orthogonal frequency division multiplexing (OFDM) systems were developed.
OFDM systems separate the transmission channel into several sub-carrier frequencies. Data is then transmitted on each of those sub-carriers in parallel. The symbols are spread across the different carriers and each of them occupies only a small portion of the frequency band; however, the symbols are relatively long in duration. This parallelism allows the symbol duration for a given data rate to be extended, thus providing immunity to dispersive fading which would otherwise cause intersymbol interference. This immunity, which leads to greatly simplified equalization schemes to compensate for channel effects, motivated many bodies, among them the IEEE 802.11 and the ETSI HIPERLAN committees, to adopt OFDM through their wireless standards.
Unfortunately, OFDM systems present many obstacles for practical and reliable operation. OFDM systems are particularly sensitive to time and frequency synchronization issues. For reliable operation, the receiver ideally should know not only the exact position of each frame of the received signal in time, but also the transmitter's oscillator frequency. Transmitter and receiver radio frequency oscillators are usually not perfectly frequency locked. In fact, both of the afore-mentioned IEEE 802.11a and ETSI HIPERLAN/2 standards state that a given oscillator can deviate by a maximum of 20 ppm from the carrier frequency associated with a given channel, thus generating a carrier frequency offset (CFO). OFDM systems are especially sensitive to CFO. Virtually all receivers use a downconverter before A/D conversion in order to subsequently perform digital signal processing at baseband. The received signal is then represented as an amplitude as well as a phase, relative to the oscillator, in the complex plane. The CFO generates a linearly increasing phase offset for each time sample at baseband. Given knowledge of that phase offset, the CFO can be corrected simply by applying the opposite offset to the received time samples.
Synchronization and CFO compensation techniques are known which exploit known periodically-repeating signals added at the beginning of radio packets transmitted in OFDM systems, forming part of the so-called preamble of such packets. Packets in OFDM systems conforming to IEEE 802.11a and the ERP-OFDM clause of IEEE 802.11g standards exhibit this structure. These standards specify a physical layer convergence protocol (PLCP) preamble that is used for synchronization and CFO compensation purposes. In receivers equipped with multiple antennas, the preamble can also be used for channel gain and phase estimation to support subsequent combining of signals from said multiple antennas.
FIG. 2 details the structure of the preamble that is prepended to the signal and data symbols in each OFDM packet under 802.11a and 802.11g/ERP-OFDM standards. Symbols t1 through t10 of the preamble comprise short training symbols and symbols T1 and T2 of the preamble comprise long training symbols. The short symbols are all identical, and together form the short symbol training sequence (herein referred to as SSTS), which is periodic with a period of 800 ns. Likewise, the long training symbols T1 and T2 are identical and, together with guard interval GI2, which consists of a copy of the latter half of a long symbol, form the long symbol training sequence, (herein referred to as LSTS). Together, the SSTS and LSTS are intended to serve the functions of signal presence detection, automatic gain control (AGC), diversity selection, coarse and fine CFO estimation and compensation, timing synchronization, and channel estimation for equalization and diversity combining purposes.
Most time synchronization algorithms for OFDM WLANs are based on either auto-correlation (correlation of the received signal with itself, exploiting known statistical properties of the transmitted signal) or cross-correlation (correlation of the received signal with a known training sequence) techniques. Both techniques usually exploit the short, periodic training sequence preceding each frame (the SSTS). In the 802.11 and HIPERLAN/2 OFDM frame structure, for example, the SSTS is actually a repetition in time of the same 16 samples 10 times, assuming the typical 20 Mhz sample rate. In the widespread, so-called maximum-likelihood (ML) approach, the periodic nature of the SSTS is exploited through auto-correlation techniques, widely used since they can not only perform time synchronization but also frequency synchronization. The auto-correlation function R(τ) is usually computed for τ=16, the period of the short sequence at the typical sampling rate of 20 Mhz in 802.11a and 802.11g implementations.
Unfortunately, time synchronization based on the auto-correlation function exhibits poor performance in the presence of a Rayleigh fading dispersive channel. Cross-correlation techniques are used instead to efficiently perform time synchronization in such channels. Cross-correlation techniques are based on the low correlation of a period of the short preamble with a time-shifted version of itself. This is a typical, desirable property of most training sequences used in various communication systems (not restricted solely to OFDM modulation). In other words, the autocorrelation function is of a single period of the training sequence, designated as a short training symbol (in this case a 16-sample sequence), approximates a Kronecker delta function which takes on a maximum value at a delay of 0, and is equal to 0 everywhere else. However, known techniques using cross-correlation before carrier frequency offset has been corrected are not entirely satisfactory because the CFO leads to ambiguity.
Examples of US patent documents which disclose synchronization algorithms for OFDM WLANs are as follows:
U.S. Pat. No. 7,039,000 (You et al., 2006) first performs a coarse timing estimate exploiting the autocorrelation technique using R(80) rather than R(16). Received samples are then interpolated to perform a fine oversampled timing estimate using again the autocorrelation technique.
U.S. Pub. Appln. No. 2004/0047368 (Xu) exploits both a matched filter and the auto-correlation function through a hybrid design. Basically, both functions are computed and a decision is made through a criterion based on a weighted sum of the two results. Neither frequency synchronization nor channel estimation is performed through the proposed design.
U.S. Pub. Appln. No. 2006/0014494 (Vanderperren et al.) also exploits both the auto-correlation and cross-correlation techniques. CFO as well as a coarse timing estimate are first obtained through the auto-correlation function. The CFO-corrected signal is then cross-correlated to acquire a fine timing estimate.
Both of these published applications are based on the special training symbols technique mentioned by Richard Van Nee and Ramjee Prasad in “OFDM for Wireless Multimedia Communication,” Artech House, 1999, 260 pages. The technique was retargeted specifically at OFDM WLANs by Yong Wan& Ge Jian-hua, Bo Ai and Li Zong-Qiang in “A Novel Scheme for Symbol Timing in OFDM WLAN Systems”, IEEE International Symposium on Communications and Information Technology, through a relatively heavy design requiring an especially large filter. No CFO estimate is performed as only the real part of the baseband received signal is exploited, also resulting in poor performance of the design.
U.S. Pat. No. 7,039,140 (Reagan et al., 2006) exploits the structure of the long preamble to perform synchronization. A large inner product must be computed for each sample suspected of being the boundary between the short and the long training symbols. As a result, the preferred embodiment supposes a coarse timing estimate to limit the number of inner products to be performed and target a real system. More particularly, U.S. Pat. No. 7,039,140 discloses a procedure for acquiring synchronization in such an OFDM system using autocorrelation between different batches of N samples in the LSTS. This exploits the periodicity of the two halves of the groups T1 and T2 in FIG. 2, a feature that is specific to the LSTS.
A drawback is that Reagan et al. require a rough timing reference before they start and obtain the synchronization retroactively insofar as they use information received after the end of the short training sequence to locate the end of the short training sequence.
It would be desirable to achieve at least the time synchronization using only the short training sequence so that the long symbol training sequence in the same preamble can be used for other purposes, such as gain control and channel selection.
An object of the present invention, at least according to some aspects, is to at least mitigate drawbacks of such known signal acquisition techniques, or provide an alternative.
In addressing this object, aspects of the invention are predicated upon the fact that, when correlated with a period of itself, the SSTS generates a distinct impulse sequence exploitable for time synchronization. Cross-correlation is simply performed through the use of a filter matched to several periods of the short preamble. This characteristic further improves the performance of the cross-correlation techniques as it provides, to some extent, immunity to Gaussian noise.
Time and frequency synchronization are not the only challenges faced by today's designers of OFDM systems as pressure on the limited radio spectrum increases. This pressure is two-fold: the number of users is growing dramatically, and the services offered and being developed are increasingly demanding in bandwidth. Antenna arrays and associated signal processing constitute the single most promising avenue to augment both user-capacity and throughput-capacity in wireless networks.
Antenna array processing is typically performed through a linear weight-and-sum operation. When the channel is approximated as narrow band, the baseband equivalent signal y at a given time is obtained from the received N×1 vector r of baseband signals on N antennas asy=wHr where wH is the Hermitian transpose of the N×1 complex vector w, usually computed through a narrow band channel estimate on each antenna for a given user as well as potential interferers.
Another object of the present invention, at least according to certain aspects, is to at least mitigate drawbacks of known signal acquisition techniques, or provide an alternative, in the context of radio receivers employing array antennas.
According to a first aspect of the invention, there is provided a method of signal acquisition in a communications receiver for receiving Orthogonal Frequency Division Multiplex (OFDM) signals comprising data packets each comprising a preamble training sequence having a periodic structure, the method comprising the steps of
(i) sampling a received OFDM signal to obtain received-signal samples;
(ii) filtering said samples using a matched finite impulse response (FIR) filter having an impulse response matched to said periodic structure of said preamble training sequence;
(iii) detecting occurrence of a maximum amplitude of the cross-correlation output from said FIR over a predetermined number of periods corresponding to at least part of said training sequence; and
(iv) using one or more of timing, magnitude and phase of said maximum amplitude to determine time synchronization, carrier frequency offset or channel estimation.
According to a second aspect of the invention, there is provided a method of signal acquisition in an OFDM radio receiver having an array antenna comprising a plurality of antenna elements each for receiving an individual received signal comprising data packets each comprising a preamble training sequence having a periodic structure, the method comprising the steps of:
for each antenna element:
(i) sampling its received individual radio signal to obtain a series of received-signal samples;
(ii) filtering said samples using a matched finite impulse response (FIR) filter having an impulse response matched to said periodic structure of said preamble training sequence;
(iii) detecting a series of peaks at the output of said filter, each peak corresponding to one received OFDM symbol of the preamble training sequence;
(iv) detecting a maximum amplitude of the cross-correlation output among said peaks from said FIR; and
(v) determining a channel estimate (CE) as the magnitude and phase of said maximum amplitude; and
(vi) using said magnitude and phase to weight corresponding data for that antenna element in a subsequent step of combining the individual received-signals of the antenna elements, respectively.
(vii)(a) extracting the phase difference between two of said peaks,
(vii)(b) calculating from said phase difference the phase shift induced per sample by carrier frequency offset, said phase difference being a multiple of said phase shift with the multiplication factor between the two being determined by the separation of the two peaks;(viii) adjusting each phase shift estimate by adding such estimates for all antenna elements, each being previously weighted by the corresponding channel magnitude determined in step (v);(ix) the adjusted phase shift estimate being used for carrier offset correction during subsequent processing steps.
According to a third aspect of the invention, there is provided a method of signal acquisition in an OFDM radio receiver having an array antenna comprising a plurality of antenna elements each for receiving an individual received signal comprising data packets each comprising a preamble training sequence having a periodic structure, the method comprising the steps of: for each antenna element:
(i) sampling its individual received signal to obtain a series of received-signal samples;
(ii) filtering the preamble samples using a matched finite impulse response (FIR) filter having an impulse response matched to said periodic structure of said preamble training sequence;
(iii) detecting a series of peaks at the output of said filter, each peak corresponding to one received OFDM symbol of the preamble training sequence;
(iv) detecting a maximum amplitude of the cross-correlation output among said peaks from said FIR; and
(v) providing the timing of said maximum amplitude on any given antenna, or any combined signal created by combining the filter outputs at all antennas, as a timing reference for demodulation of said received data.
(vi) determining magnitude and phase of said maximum amplitude; and
(vii) using said magnitude and phase to weight corresponding data for that antenna element in a subsequent step of combining the individual received-signals of the antenna elements, respectively.
(viii)(a) extracting the phase difference between two peaks, where said two peaks can be two consecutive peaks or can be more widely separated;
(viii)(b) calculating from said phase difference the phase shift induced per sample by the carrier frequency offset, said phase difference being a multiple of said phase shift with the factor between the two being determined by the separation of the two peaks;(ix) adjusting a phase shift estimate by adding such estimate for all antennas, each being previously weighted by the corresponding channel magnitude determined in step (vi); and(x) the adjusted phase shift estimate being used for carrier offset correction during subsequent processing steps.
According to a fourth aspect of the invention, there is provided a method of signal acquisition in an OFDM (Orthogonal Frequency Domain Multiplexed) receiver for receiving received signals comprising data packets each comprising a preamble training sequence having a periodic structure, comprising the steps of;
(i) sampling a received OFDM radio signal to obtain a series of received-signal samples;
(ii) filtering the preamble samples using a matched finite impulse response (FIR) filter having an impulse response matched to said periodic structure of said preamble training sequence, said filtering being preceded by a complex multiplication by a correction coefficient for dynamic carrier frequency offset compensation, said coefficient being initially equal to 1;(iii) detecting the first two peaks at the output of said filter, each peak corresponding to one received short OFDM symbol;(iv)(a) extracting the phase difference between said two peaks;(iv)(b) calculating from said phase difference the phase shift induced per sample by the carrier frequency offset, said phase difference being a multiple of said phase shift with the factor between the two being determined by the separation of the two peaks;(v) multiplying the correction coefficient at the multiplier ahead of the matched filtering by a complex value of unit amplitude whose phase is minus the phase shift found in step (iv);(vi) pursuant to the filtering step, updating the correction coefficient for every received peak as a function of the last received two peaks according to steps (iii)-(v);(vii) detecting a maximum amplitude of the cross-correlation output among said peaks from said FIR; and(viii) providing the timing of said maximum amplitude as a timing reference for demodulation of said received data;(ix) providing the final value of the correction coefficient as a measure of the carrier frequency offset for its compensation in subsequent steps.
According to a fifth aspect of the invention, there is provided a method of estimating and correcting carrier frequency offset and estimating from received data in an OFDM radio receiver characteristics of a transmission channel whereby said received data was received, said OFDM radio receiver having an array antenna comprising a plurality of antenna elements each for receiving an individual received signal comprising data packets each comprising a preamble training sequence having a periodic structure, the method comprising the steps of:
for each antenna element:
(i) sampling a received OFDM radio signal to obtain a series of received-signal samples;
(ii) filtering the preamble samples using a matched finite impulse response (FIR) filter having an impulse response matched to said periodic structure of said preamble training sequence, said filter being preceded by a complex multiplication by a correction coefficient for dynamic carrier frequency offset compensation, said correction coefficient being initially equal to 1;(iii) detecting the first two peaks at the output of said filter, each peak corresponding to one received short OFDM symbol;(iv) determining magnitude and phase of the maximum amplitude out of the said two peaks, for each antenna, said magnitude and phase constituting the current estimate of the channel coefficient associated with each antenna;(v)(a) extracting the phase difference between said two peaks on each antenna;(v)(b) calculating from said phase difference the phase shift induced per sample by the carrier frequency offset, said phase difference being a multiple of said phase shift with the factor between the two being determined by the separation of the two peaks;(vi) expressing said phase shift on each antenna by a complex coefficient of unit magnitude, computing the weighted normalized sum of all such coefficients for all antennas, where the weight associated with a specific coefficient is proportional to the current channel gain estimate on the same antenna;(vii) multiplying the correction coefficient at the multiplier ahead of the matched filter by a complex value of unit amplitude whose phase is minus the phase shift found in the to normalized sum operation of step (vi);(viii) pursuant to the filtering operation, updating the current channel estimate and the correction coefficient for every received peak as a function of the last received two peaks according to steps (iii)-(vii)(ix) detecting a maximum amplitude of the cross-correlation output among said peaks from said FIR; and(x) determining magnitude and phase of said maximum amplitude; and(xi) using said magnitude and phase to weight corresponding data for that antenna element in a subsequent step of combining the individual received-signals of the antenna elements, respectively;(xii) providing the final value of the correction coefficient as a measure of the carrier frequency offset for compensation therefor in subsequent steps.
According to a sixth aspect of the invention, there is provided a method of jointly A—estimating and correcting carrier frequency offset, B—recovering timing (synchronizing) and C—estimating from received data in an OFDM radio receiver characteristics of a transmission channel whereby said received data was received, said OFDM radio receiver having an array antenna comprising a plurality of antenna elements each for receiving an individual received signal comprising data packets each comprising a preamble training sequence having a periodic structure, the method comprising the steps of:
for each antenna element:
(i) sampling its individual received OFDM radio signal to obtain a series of received-signal samples;
(ii) filtering the preamble samples using a matched finite impulse response (FIR) filter having an impulse response matched to said periodic structure of said preamble training sequence, said filtering being preceded by a complex multiplication by a correction coefficient for dynamic carrier frequency offset compensation, said correction coefficient being initially equal to 1;(iii) detecting the first two peaks at the output of said filter, each peak corresponding to one received OFDM symbol of the training sequence;(iv) determining magnitude and phase of the maximum amplitude out of the said two peaks, for each antenna, said magnitude and phase constituting the current estimate of the channel coefficient associated with each antenna;(v)(a) extracting the phase difference between said two peaks on each antenna;(v)(b) calculating from said phase difference the phase shift induced per sample by the to carrier frequency offset, said phase difference being a multiple of said phase shift with the factor between the two being determined by the separation of the two peaks;(vi) expressing said phase shift on each antenna by a complex coefficient of unit magnitude, computing the weighted normalized sum of all such coefficients for all antennas, where the weight associated with a specific coefficient is proportional to the current channel gain estimate on the same antenna;(vii) multiplying the correction coefficient at the multiplier ahead of the matched filter by a complex value of unit amplitude whose phase is minus the phase found in the normalized sum operation of step (vi);(viii) pursuant to the filtering operation, updating the current channel estimate and the correction coefficient for every received peak as a function of the last received two peaks according to steps (iii)-(vii)(ix) detecting a maximum amplitude of the cross-correlation output among said peaks from said FIR; and(xi) determining magnitude and phase of said maximum amplitude;(xii) using said magnitude and phase to weight corresponding data for that antenna element in a subsequent step of combining the individual received-signals of the antenna elements, respectively,(xiii) providing the timing of said maximum amplitude on any given antenna, or any combined signal created by combining the filter outputs at all antennas, as a timing reference for demodulation of said received data; and(xiv) providing the final value of the correction coefficient as a measure of the carrier frequency offset for its compensation in subsequent steps.
Other aspects of the invention are OFDM receivers as specified in the independent claims.
Features of the different aspects are specified in the corresponding dependent claims.
Preferred embodiments of the invention will now be described with reference to the accompanying drawings.